The present invention relates to an apparatus for sealing the opening between an inner shaft and an outer tube, such as the opening between a a propeller shaft and a stern tube on a ship.
Typically, a propeller shaft of a ship is enclosed in a stern tube. The stern tube, including any portion thereof inside the hull of the ship, is full of water. An inner seal, such as a packing gland or a syntron seal assembly, at the fore end of the stern tube prevents the water from freely entering the ship. Any disruption in the water-tight integrity of the stern tube, or the inner seal, can lead to the influx of great quantities of water. Increased water pressure further exacerbates this problem. For example, on some ships, the stern tube is at least 30 feet under water, and the water pressure at such level forces water quickly through any breach.
To make emergency repairs or to conduct normal preventative maintenance on the inner seal, it is necessary to evacuate the water from the stern tube. This evacuation is referred to as "dewatering" the stern tube. Routine repairs on smaller vessels may be accomplished by hauling them out of the water on a marine railway. However, for large vessels requiring these repairs, only large graving docks, or floating dry docks, are capable of handling the size and weight, and these facilities are most often not available immediately. Additionally, dry docking is extremely expensive and time-consuming. Therefore, when ship scheduling or an emergency prohibits waiting for a dry dock, the external opening of the stern tube is sealed by any means available, and the stern tube is dewatered by loosening the inner seal and allowing the water to drain inside the ship. This draining of salt water into the inside of the ship is highly undesirable and much detested by the ships' masters in that it not only places large quantities of a corrosive liquid into a metal environment, but also can be dangerous. If the external seal fails, then water can flood quickly into the ship.
Some diving teams have effectively dewatered stern tubes from outside the ship by forcing a smashed pipe into the space between the propeller shaft and the inside surface at the aft face of the stern tube prior to sealing the external opening of the tube, and then utilizing an eductor head for drawing the water out of the stern tube. However, this method of dewatering also requires that a vent be provided, other than the suction pipe, in order for outside air to flow into the enclosed stern tube to replace the water that is being removed. Thus, this method also requires that the inner seal be loosened, or that some other opening be established between the inside of the ship and the inside of the stern tube in order to provide a vent through which air can freely pass. On some occasions a second pipe has been smashed into the space between the shaft and the inside surface at the aft face of the stern tube to enable air to be vented into the tube. However, the small space available in that location severely restricts the size of pipes that may be used; which results in unacceptably lengthly dewatering times.
In the prior art, the external opening of the stern tube usually has been sealed by the application of a composite of materials typically including wooden wedges, bundles of line or rope, rags, heavy neoprene tape, "wet hardening" epoxy, and the like, or, with specially constructed cofferdams. The cofferdam must be designed to fit around the propeller shaft and the stern tube and against the hull to form a water-tight seal, with calking typically used to fill any small openings at the edges of the cofferdam.
On destroyers of the Spruance class, the port side stern tube has a lower portion which terminates in an aft face that is approximately perpendicular to the longitudinal axis of the tube, and an upper portion having a concave inner surface which extends further aft. Some of the tube aft faces are slanted, either fore or aft, and some of them are truly perpendicular. Still others have corrosion problems in the termination area resulting in the loss of sizable pieces of material from the aft face of the tube which prohibit sealing the exterior opening with a contact gasket positioned against the aft face. No known cofferdams exist for sealing the exterior opening of the port side stern tube on a Spruance class destroyer.
An extremely large cofferdam would be required on such ships due to the unusual configuration of the stern tube and hull at the port propeller shaft exit point, making it necessary that the cofferdam extend from forward of the forward end of the stern tube barrel, to aft of the aft end of the upper portion of the stern tube that extends aft. The overall distance is a minimum of 12 feet.
A cofferdam of sufficient size and strength to safely seal the external opening between the propeller shaft and stern tube on such ships would be so large and bulky as to require a heavy crane for topside (surface) handling, plus an oversized diving crew for handling it underwater where no crane can be placed to help. Additionally, the weight off such a cofferdam would pose a serious hazard to the divers attempting to control its movement in a confined area between ship's hull, stern tube and propeller shaft while performing the installation.
The prior art method of applying composite materials to seal the exterior opening of the stern tube typically takes two or three days. In addition, such applications of composite materials have collspsed on some occasions and developed substantial leaks on other occasions after the inner seal to the stern tube has been removed and thereby placed the entire ship in jeopardy.